Gas turbines typically include a compressor, a combustion section downstream from the compressor and a turbine downstream from the combustion section. The combustion section includes at least one combustor that is at least partially enclosed by an end cover that is coupled to an outer casing. The outer casing at least partially defines a plenum around the combustor. At least one fuel nozzle extends downstream from the end cover and at least partially through a cap assembly that extends radially within the casing. An annular liner such as a combustion liner, a transition duct or a transition nozzle extends downstream from the cap assembly. The liner generally includes a plurality of heat transfer features such bumps, ridges, ribs or grooves that extend along an outer surface of the liner. The liner at least partially defines a combustion chamber within the combustor. The liner may also at least partially define a hot gas path that extends between the combustion chamber and an inlet of the turbine.
An annular flow sleeve such as a combustion liner flow sleeve and/or an impingement sleeve surrounds the liner. An annular flow passage is at least partially defined between the outer surface of the liner and an inner surface of the flow sleeve. The annular flow passage at least partially defines a flow path between the plenum and a head end of the combustor which is upstream from the cap assembly. The flow sleeve generally includes a plurality of cooling holes which provide for fluid communication between the plenum and the annular flow passage.
In operation, air enters the compressor through an inlet and is progressively compressed as it flows through the compressor towards the combustion section. The compressed air flows from the compressor into the plenum at a first pressure which is commonly referred to as the compressor discharge pressure. A portion of the compressed air flows through the cooling holes of the flow sleeve and into the annular flow passage. The compressed air is routed through the annular flow passage towards the end cover or head end of the combustor. The compressed air reverses direction at the head end and is routed through or across each fuel nozzle. Fuel from each or some of the fuel nozzles is mixed with the compressed air to form a combustible mixture. The combustible mixture is routed into the combustion chamber where it is burned to produce a hot gas at a highly energized state. The hot gas flows through the hot gas path to the turbine.
The compressed air that is routed through the annular flow passage provides convective and/or conductive cooling to the outer surface of the liner. However, due to friction with the outer surface of the liner and/or an inner surface of the flow sleeve, a significant pressure drop is realized at the head end of the combustor with respect to the compressor discharge pressure. As a result, the pressure of the compressed air that flows through a premix flow passage defined within a burner tube that surrounds a portion of each fuel nozzle may not be sufficient to prevent flame holding at or near a tip portion of the fuel nozzles and/or within the burner tube, thereby increasing thermal stresses at the tip portion and/or limiting the mechanical life of the burner tubes and/or the fuel nozzles and potentially causing damage to surrounding combustor parts and/or to the hot gas path.
In particular combustor designs having a center fuel nozzle surrounded by one or more secondary fuel nozzles comprising one or more tube bundles, also known as a micro mixer system, the center fuel nozzle limits the flame holding capability of the micro mixer system. This is at least partially due to a significant pressure drop of the compressed air at the head end of the combustor with respect to the compressor discharge pressure. This pressure drop is generally caused by friction loses through the annular flow passage, friction losses due the plurality of tubes of the tube bundles and/or additional losses due to a large portion of the compressed air being directed through each of the plurality of tubes. Accordingly, an improved system for providing compressed air to a fuel nozzle, particularly a center fuel nozzle of a combustor would be useful in the art.